1. Field of the Invention
This invention relates to apparatus for distributing gas and liquid simultaneously from a common main conduit to filter underdrains for backwash of filter media.
2. Description of the Prior Art
In a typical gravity filter for filtering water and wastewater, one or more filter tanks are arranged in adjacent or opposing batteries. Filter underdrain laterals are laid on the floor of the filter tank one next to the other in parallel rows to define gas and liquid flow conduits below a bed of filtering media. The conduits make possible the collection of filtered liquid during filtration and the distribution of gas and liquid for backwash. A common main conduit (known as a "flume") is located immediately next to the filter tank to collect the filtered liquid from the underdrain laterals during filtration and to distribute gas or liquid to the underdrain laterals during gas (typically air) or liquid (typically water) backwash. The common main conduit also distributes gas and liquid simultaneously to each of the underdrain laterals during concurrent gas and liquid backwash. In some installations, the common main conduit has its floor located somewhat lower than the elevation of the filter floor (see FIG. 2) or even on the same elevation as the filter floor (see FIG. 1). Consulting engineers and contractors prefer these arrangements, known as "recessed flume" and "flat bottom flume", because they are easier and cheaper to install. In retrofitting old filters with new underdrains, it is preferable to maintain the existing flume structure to reduce costs.
The interface of gas and liquid in the underdrain laterals during concurrent gas and liquid backwash is only a few inches above the filter floor. The interface of gas and liquid in the common main conduit at this time would be even lower than that inside the laterals due to friction loss through the wall sleeves. The wall sleeves connect the common main conduit to the underdrain laterals. There is, therefore, practically no room in flat bottom flumes and insufficient room in some recessed flumes for distribution of liquid to the underdrain laterals during concurrent gas and liquid backwash. In these cases, the air space is disproportionately large in the common main conduit.
In order to provide near uniform distribution of liquid to the underdrain laterals, the maximum flow velocity of liquid inside the common main conduit should be limited to two feet per second or lower. Generally, the smaller the cross-sectional area for the flow path of liquid, the greater the velocity. Since the floor of the conduit is on the same elevation as the filter floor in flat bottom flumes, and the floor of some flumes may be insufficiently recessed, the only way to provide a greater cross-sectional area for the flow of liquid to keep maximum velocity at less than two feet per second is to raise the interface of gas and liquid from the floor or deepen the flume recess. If the flume is insufficiently recessed, as shown in FIGS. 8 and 9, passage for the gas to the underdrain laterals may be blocked by the liquid during concurrent gas/liquid backwash.
The alternatives are to provide adequate room below the interface of gas and liquid inside the common main conduit by lowering the floor elevation for liquid distribution (see FIG. 3), or by providing separate means for the controlled conveyance and distribution of gas and liquid (see FIGS. 4, 5, 6 and 7).
In the case of FIG. 3, extra excavation, form work and concrete structural work are required. This is very expensive for both retrofit jobs and new construction. In addition, baffles or individual wall sleeves with lateral inlet elbows are required to direct liquid to the laterals. In the case of FIGS. 4, 5, 6 and 7, a stainless steel gas header with tubular branches to each individual underdrain lateral is very expensive to fabricate and install. The individual gas tubes from the gas header to each underdrain lateral, shown in FIGS. 6 and 7, may cause cross connection problems between unfiltered and filtered water if leaks occur, since the individual tubing must go through the filter media bed.
It is, therefore, an object of this invention to create more room inside the common main conduit by raising the interface of gas and liquid in a unique manner during concurrent gas and liquid backwash, while at the same time providing clear passages inside the common main conduit for gas flow to the underdrain laterals. This is to be accomplished without raising the overall dynamic pressure within the backwash system.